IMAGEGUIDED IMRT FOR LOCALIZED PROSTATE CANCER WITH DAILY REPOSITIONING : ASSESSING THE DIFFERENCE B

1
IMAGE-GUIDED IMRT FOR LOCALIZED PROSTATE CANCER
WITH DAILY REPOSITIONING ASSESSING THE
DIFFERENCE BETWEEN PLANNED DOSE AND DELIVERED
DOSE DISTRIBUTION

• A ARNAUD 1, G CREHANGE 1, JP BRENIER 1, K
  PEIGNAUX 1, G TRUC 1, E LAGNEAU 1, C DEVILLE 2, F
  BONNETAIN 2, JL DUMAS 3, P MAINGON 1
• 1 Department of Radiation Oncology, Centre
  Georges François Leclerc, 21000 Dijon, 2
  Department of Biostatistics, Centre Georges
  François Leclerc, 21000 Dijon, 3 Department of
  Radiation Oncology, University Hospital Jean
  Minjoz, 25000 Besançon

Purpose Technological advances such as
Intensity-Modulated Radiation Therapy (IMRT) have
yielded significant gains in tumor control and
reduced toxicity. Continuing advances have
focused on the characterization and control of
patient movement, organ motion and anatomical
deformation, which all introduce geometric
uncertainty. For these reasons, combining
image-guided and intensity-modulated radiation
therapies (IG-IMRT) is becoming a crucial
requirement for further innovation in conformal
radiotherapy, to ensure that the prescribed dose
of radiotherapy is delivered as planned into the
target. We aimed to investigate the dosimetric
benefit of on-line repositioning with Ultra
Sound-based system during a full course of
intensity-modulated radiation therapy (IMRT).

• Results

• The differences observed between these dosimetric
  parameters for OAR were not significant

• Table 2 Dosimetric comparisons between plan A
  and plan B according to selected ICRU criteria
  (n 20)

• Methods Material
• Between May 2003 and March 2006
• twenty patients with localized prostate cancer
• IG-IMRT to 78 Gy at 2 Gy per fraction in 39
  fractions, over 53 days
• Figure 1 Beam
  arrangement for first sequence from 0 to 46 Gy
  (Figure 1A) and second sequence from 46 to 78 Gy
  (Figure 1B)
• Dosimetric CT were delineated by 2 referent
  physicians using 2 different prostate margin
  definitions 5 mm or 10 mm in all directions
  except posteriorly where the margin was 5 mm for
  all the patients
• Daily isocenter shifts using the ultra-sound
  based repositioning system (SonArray, Varian)
  were implemented in a post-treatment plan sum,
  resulting in a total of 780 recalculated daily
  dosimetric analysis (Plan B)
• These post-treatment plans were compared to the
  20 pre-treatment plans, which assume no shift of
  the isocenter of the prostate (Plan A)

• The results of EUD calculation for plan A and
  plan B were significant for P-CTV and P-PTV. For
  P-CTV we found a mean EUD value equal to 78.13 Gy
  for plan A and 75.87 Gy for plan B

Table 3 Equivalent Uniform Dose (EUD)
comparisons between plan A and plan B (n 20)
1A
1B
Figure 2B post-treatment plan sum, resulting in
a total of 780 recalculated daily dosimetric
analysis, which is assimilated (Plan B).
Figure 2A pre-treatment plans, wich assume no
shift of the isocenter of the couch ( plan A )

• Both plans were compared using the following
  ICRU criteria
  
• - D95 (dose to 95 of the prostate), D98,
  D50, D mean, for prostate CTV (P-CTV) and PTV
  (P-PTV)
• - D mean, D98, V70 (volume receiving 70
  Gy) and V75 for organs at risk (OAR) studied
  rectum and bladder
• Wilcoxon tests were performed with a p-value ?
  0.01 as significant

• For plan A and B margin did not impact on all
  described criteria for P-PTV, EUD and OAR

Table 4 Comparisons between a P-PTV with 5mm
margin vs 10 mm for plan A (with on-line
repositioning).and plan B (without repositioning)
(n 10)

• Results
• According to the UICC 2002 classification,
  patients (pts) were staged as follows T1b 2
  pts, T1c 12 pts, T2a 3 pts, T2b 2 pts, T2c
  1 pt
• Median Gleason score was 6 ( range, 3-8 )
• Mean initial PSA value was 11.1 ng/ml ( range,
  2.4-45.0 ng /ml )
• Ten pts were treated with a 5 mm margin and 10
  pts with a 10 mm margin
• For P-CTV and P-PTV, all ICRU criteria resulted
  in a statistically significant higher delivered
  dose to the prostate or prostate margin with
  plan A compared to plan B, excepted for D2 and
  D50 for P-CTV

Table 1 Dosimetric comparisons between plan A
and plan B according to selected ICRU criteria
(n 20).
Conclusion We have retrospectively demonstrated
a statistically significant impact of on-line
repositioning on a better homogeneity of dose
distribution to the prostate and the P-PTV,
according to validated dosimetric and
radiobiologic criteria. Daily on-line
repositioning did not affect dose distribution to
OAR such as rectum and bladder. No differences
were observed either on OAR or on the dose
distribution between 5mm and 10 mm margin for
P-PTV and OAR. For selected centres were IG-IMRT
is in routine practice, the recommended margin
for PTV during a full course IG-IMRT could be 5
mm. This technique implemented in routine could
be the basis for studying the impact of IGRT by
using a cone-beam CT.
AASTRO october 2007, Los Angeles